This invention relates to an adaptive braking system for a vehicle having fluid-pressure actuated brakes.
Adaptive braking systems for vehicles having fluid-pressure actuated brakes, and in particular, systems designed for use with vehicles having brakes operated by air pressure, are limited in their performance by governmental regulations which require rapid brake actuation response during brake applications when intervention of the adaptive braking system is not required. To meet these fast response requirements, the various components of the air brake system, including the relay valves which are commonly also used as adaptive braking modulators, must have relatively large fluid pressure flow rates. However, these large flow rates are detrimental to good adaptive braking system performance, because the adaptive braking system cannot respond quickly enough to prevent wheel lock in this type of system without extending the stopping distance. In order to overcome this problem, some prior art systems have used so-called "pneumatic logic" within the modulating relay valve which is responsive to initiation of operation of the adaptive braking system to limit the presssure build rate through the modulating relay valve after the first adaptive braking cycle. Although this type of system has, in general, operated satisfactorily, the restricted flow rate limits the system response when higher pressure is required by the adaptive braking system and the increased complexity of the modulating relay valve decreases its reliability because small orifices are generally required, which are subject to interference by contaminants in the vehicle air supply. Other types of prior art systems have overcome this problem by employing a duty cycle translator which converts a brake pressure control signal into a prescribed duty cycle for a solenoid-actuated valve which is a part of the modulating relay valve which controls communication to the brake actuators during operation of the adaptive braking system. In this type of system, the solenoid is operated at a sufficiently high frequency that the relay valve will effectively integrate the duty cycle into a steady state pressure level. Although this system has also, in general, performed satisfactorily, systems of this type have a characteristically slow response which is detrimental to good adaptive braking system performance because they are open-loop systems. Theoretically, this defect may be cured by providing a pressure feedback signal by locating a pressure transducer in the modulating relay valve or brake pressure actuators and by comparing the signal generated by this transducer to the brake pressure control signal at the duty cycle translator. However, the requirement of a pressure transducer greatly increases the system cost and also increases installation problems, so this solution has not been deemed to be practical.
The present invention is related to an adaptive braking system having a duty cycle translator for controlling actuation of the modulating relay valve, and provides circuitry within the duty cycle translator which is responsive to the brake pressure control signal to generate a pseudo-pressure feedback signal which approximates the pressure level in the brake pressure actuators. This circuitry within the duty cycle translator approximates the time delays inherent in the electrically actuated portion of the modulating relay valve, and further approximates the response characteristics inherent within the modulator or relay valve and the brake actuators.